The large increase in the abuse of therapeutic agents, particularly the barbiturates, by the general population as well as military personnel, has brought with it a substantial need to improve analytical techniques for the determination of such agents in biological fluids. In many instances, medical treatment centers are faced with the immediate need for determining the identity of a barbiturate taken by a patient who is unable, being in a comatose condition, or unwilling to supply such information to the treating physician. Early procedures involved the identification of barbiturates by extraction and thin-layer, gas chromatographic and spectrophotometric methods. These techniques have the disadvantages of being relatively time-consuming, laborious and lacking great sensitivity. Recently, a rapid and sensitive immunoassay procedure involving the reaction between antibodies and barbiturate antigen was described by S. Spector in U.S. Pat. No. 3,766,162 and by S. Spector and E. J. Flynn in Science, 174, 1037 (1971). This procedure, however, requires sophisticated and expensive equipment, such as scintillation counters. Therefore, it would be desirable to develop a rapid and highly sensitive assay for detecting the presence of barbiturates in biological fluids which would not require sophisticated equipment and could be easily performed on site by laboratory technicians having a minimum of training.